Previously, we constructed a humanized antibody (HuS10) that binds to the common a antigenic determinant on the S protein of HBV. In this study, we evaluated its HBV-neutralizing activity in chimpanzees. A study chimpanzee was intravenously administered with a single dose of HuS10, followed by intravenous challenge with the adr subtype of HBV, while a control chimpanzee was only challenged with the virus. The result showed that the control chimpanzee was infected by the virus, and thus serum HBV surface antigen (HBsAg) became positive from the 14th to 20th week and actively acquired serum anti-HBc and anti-HBs antibodies appeared from the 19th and 23rd week, respectively. However, in the case of the study chimpanzee, serum HBsAg became positive from the 34th to 37 th week, while actively acquired serum anti-HBc and anti-HBs antibodies appeared from the 37th and 40th week, respectively, indicating that HuS10 neutralized the virus in vivo and thus delayed the HBV infection. This novel humanized antibody will be useful in the immunoprophylaxis of HBV infection.